1. Field of the Invention
The present invention relates to an automatic focusing device, adapted for use in a camera or the like, and capable of automatic focusing to a moving object.
2. Related Background Art
For driving a phototaking lens for focusing to a moving object without delay, there is already known a technology called moving object prediction or prediction drive.
As an example, in an automatic focusing device disclosed in the Japanese Laid-Open Patent Appln. No. 60-214325 of the present assignee, the phototaking lens is driven by prediction of future lens position, based of the latest defocus amount and plural defocus amounts in the past obtained from focus detecting means.
Such an automatic focusing device, for predicting the future lens position based on the latest defocus amount and the plural past defocus amounts, is capable of providing a highly precise prediction and effecting exact lens driving if the lens position varies substantially linearly in time for the moving object. However, the prediction cannot be conducted accurately if the lens position does not vary linearly, so that the lens may show overrun or underrun with respect to the real in-focus position.
For example, if an automobile moving at a constant speed as shown in FIG. 1 is followed by a camera at a shortest distance S, the focusing lens ideally shows a movement in time indicated by a solid line L in FIG. 2.
FIG. 2 shows the lens position Z(t) in the ordinate as the lens shifts from a position corresponding to the infinite object distance, as a function of time t in the abscissa, wherein a lens position Zs corresponds to said closest position S.
FIG. 3 shows the result of linear prediction in a conventional device, for the object shown in FIG. 1 providing the lens shift pattern shown in FIG. 2.
In FIG. 3, a broken line M indicates the lens position as a function of time under control by the conventional technology. As the solid line L indicating the ideal pattern shows rapid change at the lens positions Z(t1) and Z(t2), the prediction based on the past data in the conventional device results in lens overrun.
Particularly when the charge accumulating time of the sensor employed for focus detection becomes long due to the low brightness of the object, or when the exposure operation is inserted between the focus detecting operations, the interval between the detections of defocus amount becomes longer, whereby the accuracy of prediction is further deteriorated. Consequently the overrun becomes more conspicuous, and, in the worst case, the focus detection becomes impossible because the amount of movement of the object becomes too large in said interval, whereby the proper lens drive becomes impossible.
There is also known an automatic focusing device for constantly focusing, on the photographic film, the image of an object moving with a particular pattern. Such an automatic focusing device is capable of memorizing the locus of phototaking lens positions for maintaining the focused state for said particular moving pattern of the object and driving the lens according to the memorized locus.
Such known automatic focusing device, however, is subject to certain drawbacks.
In such lens driving method according to the memorized locus of lens positions, there has not been paid particular consideration to the method of entry of the memorized locus of lens positions. If the geometrical conditions (object distance, speed etc.) of the object movement are known, the locus information indicating the lens position for focusing to the moving object as a function of time can be theoretically calculated. However, as the actual object does not necessarily move exactly according to such conditions, there may result a focus error even if the phototaking lens is driven according to thus calculated locus information.
As an example, when an automobile linearly moves toward a camera, as shown in FIG. 4, with a constant speed V and a shortest distance S to the camera, the locus Z of lens position from the initial distance P can be represented as follows: ##EQU1## wherein ##EQU2## In the equation (1), f is the focal length of the phototaking lens, t is the time, and the lens position Z is derived, from known lens equations for a single lens, taking the lens position focus to the infinite distance as zero.
However, if the actual object has a closest distance Sr and a speed Vr, there will result an error between the lens locus Z obtained from the equation (1) and the actual locus Zr obtained by putting Sr, Vr into the equation (1), and the obtained image becomes out of focus if said error is large.
The conventional automatic focusing device discriminates whether the object is moving, based on plural defocus amounts obtained in the past from focus detecting means, then predicts the movement of the object based on the past plural defocus amounts and corrects the defocus amount of the focus detecting means so as to follow the movement of the object. Such technology, for example, as disclosed in the Japanese Laid-Open Patent Appln. No. 63-148218 of the present assignee, serves to maintain the in-focus state even for a moving object.
However, since such conventional technology effects the lens drive based on the past plural defocus amounts, there may result an erroneous lens drive by an error in the detected defocus amount when a person, for example, passes in front of the main object or when the main object momentarily goes out of the focus detecting frame. There may also be an overrun of the phototaking lens due to a sudden change in the lens driving direction in case of an object passing by the camera.